I'm not saying that you are wrong about generic cases, but in the case of Squid I think the author has done proper analysis before deciding on an architecture

This essay is written by Poul-Henning Kamp ( http://en.wikipedia.org/wiki/Poul-Henning_Kamp ) so he has enough experience with OS level code to comment on this particular area. I guess it can depend on what kind of problem you are trying to solve. In the case of Varnish vs Squid, his approach worked better.